A bearing to be used at, for example, a joint section of an arm of construction machinery is required to have excellent wear resistance because remarkably large contact pressure is applied to a bearing surface of the bearing. As the bearing of this type, for example, there are known a bearing obtained by cutting cast steel alloy and a bearing obtained by embedding particles of graphite in a sliding surface in a spotted manner. However, both of the bearings have a problem of high manufacturing cost. Accordingly, instead of those bearings, a sintered bearing made of sintered metal excellent in moldability is proposed. For example, in Patent Literature 1, as a bearing for construction machinery, there is disclosed a sintered bearing obtained by dispersing copper in iron-carbon-based alloy containing a martensitic structure. In this sintered bearing, the entire sintered compact is subjected to hardening after sintering, and then, inner and outer peripheral surfaces and end surfaces thereof are subjected to cutting and grinding to be finished to predetermined dimensions.
Besides the foregoing, in Patent Literature 2, there is disclosed a procedure for forming a green compact having a two-layered structure in which different materials are used on an inner peripheral surface side and an outer peripheral surface side of the sintered compact in order to use different materials in respective sites of the sintered compact so as to impart different functions to the respective sites. Specifically, the enter peripheral surface side of the green compact is formed through use of first powder having high strength, and the inner peripheral surface side is formed through use of second powder excellent in low friction property, and then, the green compact is sintered.